Services provided by a mobile communication system using a wideband Code Division Multiple Access (hereinafter, referred to as “CDMA”) scheme include packet service communication that transmits large capacity data such as packet data and circuit data and a multimedia broadcasting/communication service that can provide a multimedia service as well as a conventional voice service. In order to support the multimedia broadcasting/communication, a Multimedia Broadcasting/Multicast Service (hereinafter, referred to as an “MBMS”) in which one multimedia data source or a plurality of multimedia data sources provide a service to a plurality of user equipments is provided.
The MBMS commonly refers to a service that transmits the same multimedia data to a plurality of receivers through a wireless network. At this time, one radio channel is shared by the plurality of receivers so that radio transmission resources can be saved. The MBMS supports multimedia transmission types such as a real time image, a voice, a still image, a text message and the like. The MBMS is a service that can simultaneously transmit voice data and image data according to the multimedia transmission type, which requires a large amount of transmission resources. In the MBMS, since the same data should be transmitted to a plurality of cells where users are located, a Point to Point (hereinafter, referred to as “PtP”) access or a Point to Multiple (hereinafter, referred to as “PtM”) access is performed according to the number of users located in each cell.
Meanwhile, a service coverage in a mobile communication system can perform various wireless accesses in a mega cell, a macro cell, a micro cell, a pico cell and the like according to a service range and a transmission rate defined in the internal standard of ITU-T Q.1711, and the service coverage is different according to a type of the cells.
Further, the service coverage and a data transmission speed may differ somewhat depending on a mobile communication service scheme, and an example of a service method according to a type of each cell will be described below.
First, the mega cell provides a wideband service that covers a wide area by using a satellite communication network, and a cell radius thereof ranges from 100 Km to 500 Km in which data and voice services can be simultaneously provided. The macro cell has a service radius within 35 Km, which is mainly used for a coverage extension of an area where subscribers' call volume is small such as a rural area and a semi-smooth area in the mobile communication system,
A coverage of the micro cell ranges from 0.5 km to 1 km, and a visibility propagation path where a mobile station can see a node B antenna becomes a main transmission path. Further, the pico cell installs an optical distributed system or a small repeater within a building to provide a service to a building-concentrated area and a building with a basement within a urban center within a radius of 50 m. The optical distributed system within the building provides an in-building service by connecting a transmission line (E1 line) from a switching center to an optical distributed node B within the building and installing an optical cable within the building and attaching an antenna to the building. The small repeater receives a terrestrial signal and provides a service to a basement of the building by using an omni antenna or a patch antenna. A data transmission rate corresponds to a maximum of 2 Mbps.
A cell service area and an application area vary depending on each service in the mobile communication, and a distance range and a subscriber acceptance capacity within the cell area are changed according to a position of the node B antenna, a height of the antenna, a beam type of the antenna, a slope of the antenna, a control of transmission power and the like.
As described above, providing a service with various divisions according to a cell scale and a transmission rate within the same area is referred to as a Hierarchical Cell Structure (hereinafter, referred to as an “HCS”). For example, when a service for the micro cell and a service for the macro cell are simultaneously provided to a particular area, it may be considered that the area configures the HCS.
FIG. 1 illustrates an example of the HCS. Referring to FIG. 1, the HCS can be designed such that a plurality of micro cells 20 overlaps in a macro cell 10 area. The MBMS may be also provided through the HCS.
In general, in the HCS system, the macro cell provides a service for a high speed mobile user and a low speed data transmission service, and the micro cell provides a service for a low speed mobile user and a high speed data transmission service.
More specifically, since the macro cell has a wide cell coverage but has a limited service capacity, it is preferable that the macro cell provides the service to the user who requires the low speed data transmission service. Further, when the micro cell provides the service to the high speed mobile user, a service efficiency may be deteriorated due to a frequent handoff. Accordingly, it is preferable that the macro cell provides the service to the high speed mobile user instead of the micro cell.
In addition, according to a cell load of the macro cell or the micro cell, a method of providing a service may be changed. That is, when a cell load of a particular micro cell is very large, a method in which the macro cell provides the service to the high speed mobile user is proposed.
Meanwhile, in connection with a method of providing the MBMS in the HCS, the micro cell provides a particular service when the number of micro cells providing the particular service is very small, and the macro cell provides the service when the number of micro cells is large.